Normally distilling a hydrocarbons-containing liquid stream in an atmospheric distillation column comprising a stripping section, a rectification section and an inlet section located between the stripping and the rectification section, comprises the steps of
(a) heating the hydrocarbons-containing liquid stream at atmospheric pressure in a furnace to a predetermined temperature, and introducing the heated stream into the inlet section of the atmospheric distillation column; PA1 (b) removing from bottom of the atmospheric distillation column a residue, removing from the top an overhead and removing from the rectification section at least one side stream; and PA1 (c) cooling at least part of at least one of the side streams and introducing the cooled side stream(s) into the rectification section at a level above the level of removal, and partly condensing the overhead and introducing the condensed fraction of the overhead into the upper end of the atmospheric distillation column. PA1 (a) heating the crude oil at atmospheric pressure to a predetermined temperature in a furnace, and introducing the heated crude oil into the inlet section of the atmospheric distillation column; PA1 (b) removing from bottom of the atmospheric distillation column a residue, removing from the top an overhead and removing from the rectification section at least one side stream; PA1 (c) cooling at least part of at least one of the side streams and introducing the cooled side stream(s) into the rectification section at a level above the level of removal, and partly condensing the overhead and introducing the condensed fraction of the overhead into the upper end of the atmospheric distillation column; PA1 (d) supplying the contaminated liquid stream containing light hydrocarbons to the inlet of a membrane unit provided with a membrane, and removing from the permeate side a permeate and from the retentate side a retentate; and PA1 (e) introducing the permeate into the rectification section and supplying the retentate to the furnace.
The hydrocarbons-containing liquid stream of step (a) is normally a crude oil.
Applicant is interested in co-distilling a contaminated liquid stream containing light hydrocarbons with the crude oil.
In relation to the present invention two contaminants are of particular interest. On the one hand hydrocarbons with a high boiling point (above 480.degree. C.) and on the other hand salts present in water droplets which are dispersed in the stream of light hydrocarbons.
Examples of hydrocarbons with a high boiling point are polynuclear aromatics, polynuclear cycloparaffins, large paraffinic hydrocarbons (waxes), and olefinic components such as polynuclear cycloolefins and large olefinic hydrocarbons specially diolefins. These high boiling point hydrocarbons are soluble in the light hydrocarbons, and the solution usually has a darker colour for example an ASTM colour of 3 or more, determined in accordance with ASTM D1500. An example of a contaminated liquid stream containing light hydrocarbons is a black condensate, which is a mixture of hydrocarbons which are sometimes produced with natural gas having an ASTM colour of 3 or more. The contaminated liquid may also include waste streams for the refinery.
The salts in the hydrocarbon streams will come from formation water or from other treatments at a refinery, examples of contaminating salts are sodium chloride, magnesium chloride, calcium chloride and iron chloride. Other salts, such as sulphates may be present as well.
In case the contaminated liquid stream would only contain contaminants in the form of high boiling hydrocarbons, the contaminated liquid stream could simply be passed directly to the furnace. And in case the contaminant is a salt, optionally in combination with high boiling hydrocarbons, the contaminated liquid stream could be indirectly passed to the furnace. Before entering into the furnace, the salt is removed in a crude oil desalter upstream of the furnace.
This is simple way of co-distilling the contaminated liquid stream. However, a disadvantage of this way of co-distilling is that the two streams have to be heated to the predetermined temperature to form a mixture of vapour and liquid that is supplied to the distillation column.
There is a further disadvantage and that is associated with obtaining the reflux in step (c). Both the light hydrocarbons of the contaminated liquid stream and the light hydrocarbons of the crude oil are vaporized and subsequently partly condensed in the rectification section. In order to condense the light hydrocarbons heat has to be removed, and that is done in step (c), and the amount of heat that has to be removed is proportional to the amount of hydrocarbons that has to be condensed. Thus the larger the amount of light hydrocarbons, the more heat has to be removed. This heat, however, has to be supplied to the hydrocarbons-containing stream before it enters into the atmospheric distillation column. Thus, if the contaminated liquid stream is mixed with the crude oil, a larger amount of heat has to be supplied. Moreover it can be the case that there is not enough heat exchange surface area available to transfer the required amount of heat.
Thus there are two drawbacks of mixing the contaminated liquid stream with the crude oil: a larger stream has to be heated, and more heat has to be removed. These larger amounts of heat that have to be transferred require larger heat exchange surface areas.